Implement -reclaimMemory API and switch to manually controlled content clearing.

ASDisplayNode and several subclasses had previously cleared memory-heavy
objects like the backing store and text layout manager when the node's
view
or layer is removed from a visible heirarchy.  This works great in any
system
that uses a "working range", where exiting the range removes the node
from
the hierarchy and reclaiming memory at that time is important.
However, for
standard UIViewController patterns (unused in Paper), this behavior
causes
highly undesirable thrashing (leading to visible flashes & wasteful
re-rendering of content).

After this change, node subclasses should implement -reclaimMemory if
they
need to perform any other cleanup besides backing store destruction
when they
leave a working range or other scenario where memory reduction is
valuable.

To trigger this behavior, calling code should use
-recursivelyReclaimMemory.

r=nadi

AsyncDisplayKit/ASTextNode.mm

@@ -194,17 +194,6 @@ ASDISPLAYNODE_INLINE CGFloat ceilPixelValue(CGFloat f)
                     fminf(ceilPixelValue(renderSizePlusShadowPadding.height), constrainedSize.height));
 }
 
-- (void)willEnterHierarchy
-{
-  CALayer *layer = self.layer;
-  if (!layer.contents) {
-    // This can happen on occasion that the layer will not display unless this
-    // set.
-    [layer setNeedsDisplay];
-  }
-  [super willEnterHierarchy];
-}
-
 - (void)displayDidFinish
 {
   [super displayDidFinish];
@@ -217,16 +206,13 @@ ASDISPLAYNODE_INLINE CGFloat ceilPixelValue(CGFloat f)
   [self _invalidateRenderer];
 }
 
-- (void)didExitHierarchy
+- (void)reclaimMemory
 {
-  // We nil out the contents and kill our renderer to prevent the very large
+  // We discard the backing store and renderer to prevent the very large
   // memory overhead of maintaining these for all text nodes.  They can be
   // regenerated when layout is necessary.
-  self.contents = nil;
-
+  [super reclaimMemory];      // ASDisplayNode will set layer.contents = nil
   [self _invalidateRenderer];
-
-  [super didExitHierarchy];
 }
 
 - (void)didLoad